Multi-path searching

ABSTRACT

A multi-path searcher comprises multiple correlators (Co, C 1 , C 2 , C 3  . . . C N ) which each process a predetermined scrambling sequence and channelisation code to carry out a correlation measurement on a received pilot signal after it has been delayed by a different time delay (D) at the input of each correlator. Multiple correlation measurements (M O , M 1 , M 2  . . . M N ) are generated and analysed (A) in relation to the time delays to identify the multi-path positions. First candidate multi-path positions are identified over a first predetermined integration period (T P1 ) and are stored and compared with second candidate multi-path positions derived from multiple correlation measurements made subsequently by the multi-path searcher over a second predetermined integration period (T P2 ) in relation to a first pilot signal. Said first and second candidate multi-path positions are confirmed or rejected to define modified multi-path positions. Said successive predetermined integration periods (T P1 , T P2 ) are spaced apart to accommodate one or more similar sets of successive integration periods in an interleaved manner, and the same multi-path searcher is used to determine multi-path positions in relation to the pilot signal received in connection with each of different cells (CELL  1,  CELL  2,  . . . CELL S). Thus, the multi-path searcher carries out a first correlation cycle (T 1 ) in relation to the pilot signal of each of different cells in succession to determine first candidate multi-paths for each, and then carries out a second correlation cycle (T 2 ) in relation to each of the pilot signals again to determine second candidate multi-paths for each to modify the respective first candidate multi-paths.

This invention relates to multi-path searching in cellular communicationsystems, especially for mobile handsets in cellular networks.

A mobile handset has to be designed to accommodate multi-path signalsthat may be present in the received signal, and for this purpose itincorporates a multi-path searcher which identifies the strongest of themulti-path signals to use. In a W-CDMA system, the Common Pilot Channel(CPICH) contains a predetermined sequence of pilot bits spread withknown channelisation codes and scrambled with known scrambling codes,and the multi-path searcher makes measurements on the CPICH signal bycorrelating this against the known CPICH scrambled chip sequence toidentify the path positions and select a sub-set to use for decoding.The multi-path searcher consists of multiple correlators which eachprocess the same scrambling sequence and channelisation code to carryout a correlation measurement on the received signal after it has beendelayed by a different time delay at the input of each correlator, themultiple correlation measurements so generated being analysed inrelation to the time delays to identify the multi-path positions. Inorder to improve the accuracy and reliability in detecting pathpositions, the correlation process is extended over an integration timesufficient to identify the path positions above the noise floor of thesignal. However, an increase in integration time results in an increasein processor/ASIC size and power consumption, which in turn leads to anincrease in the manufacturing cost of the handset

In addition, there is a requirement in the W-CDMA standard that ahandset should be able to handle multi-path signals, as described above,in connection with multi-path CPICH signals corresponding to differentcells of the cellular network, and that multi-path measurements shouldbe made in relation to a predetermined number of cells should within apredetermined time, each cell being identified by its own scramblingcode and channelisation code. Thus, if a multi-path searcher is used tomake successive multi-path measurements in relation to multiple cells,the integration time for each is limited. Alternatively, if multiplemulti-path searchers are provided to handle the multiple cells inparallel, then the manufacturing cost and power consumption isincreased.

An object of the invention is to provide multi-path searching in such amanner as to mitigate the above problems.

According to the invention, a multi-path searcher is provided whichcarries out a correlation process over a predetermined integrationperiod in relation to a first pilot signal received in connection withthe first cell of a cellular network to identify the position ofmulti-paths in the signal, characterised in that information of saidmulti-path positions over a first predetermined integration period isstored as first candidate multi-path positions and compared with secondcandidate multi-path positions derived from multiple correlationmeasurements made subsequently by the multi-path searcher over a secondpredetermined integration period in relation to the first pilot signal,so as to confirm or reject candidate multi-path positions and definemodified candidate multi-path positions.

Any uncertainty in candidate multi-path positions as measured in thefirst predetermined integration period is resolved by the candidatemulti-path positions as measured in the second and subsequentpredetermined integration periods, thus shorter integration periods canbe used to reduce hardware size and cost, while an insufficiently lowlevel of uncertainty in the measurement of candidate multi-pathpositions can still be accomodated.

By spacing said successive predetermined integration periods apart,multiple sets of integration periods can be interleaved and the samemulti-path searcher can be used to determine the multi-path positions inrelation to the pilot signal received in connection with each ofdifferent cells. Thus, the multi-path searcher carries out a firstcorrelation process in relation to the pilot signal of each of differentcells in succession to determine first candidate multi-paths for each,and then carries out a second correlation process in relation to each ofthe pilot signals again to determine second candidate multi-paths foreach to confirm or reject the respective first candidate multi-paths anddefine modified candidate multi-path positions.

Thus the multi-path searcher is able to meet the standard required inmeasuring the multi-path positions of a predetermined number of cellswithin a limited time whilst still controlling the size of the hardwareused.

The invention will now be described by way of example with reference tothe accompanying drawings:

FIG. 1 is a schematic drawing of a multi-path searcher as used accordingto the invention;

FIG. 2 is a schematic drawing showing the time delayed outputs ofmultiple correlators in FIG. 1;

FIG. 3 is a plot of the outputs of the correlators of FIG. 1 in responseto a typical CPICH signal, showing candidate multi-paths; and

FIG. 4 is a diagram showing the timing of successive candidatemulti-path measurements for different cells.

The multi-path searcher in FIG. 1 comprises multiple correlators C₀, C₁,C₂, C₃ . . . C_(N) which are all fed the same input signal generated bya cellular receiver RX. The input signal is fed to the correlators via achain of delay elements D which each introduce a delay Δ so that theinput signal is delayed to each of successive correlators C₀, C₁, C₂, C₃. . . C_(N) by a progressively increasing interval Δ, 2Δ, 3Δ . . . NΔ asshown in FIG. 2. In the first of multiple integration periods T_(P1),each of the correlators processes the input signal using a scramblingsequence for the common pilot channel CPICH of a first cell CELL 1 so asto make multiple correlation measurements M₀, M₁, M₂, M₃ . . . M_(N),which are analysed in a processor A as a time plot, shown in FIG. 3, todetermine multi-path positions in accordance with a reference L.

These multi-path positions are stored in a corresponding first store P₁.

It will be appreciated that the correlation process in each correlatorinvolves a de-scrambling, de-spreading and accumulation, of the bitstreams representing the input signal to produce one of the probabilitymeasurements M₀, M₁, M₂, M₃ . . . M_(N). The reference level L is set ata predetermined level corresponding to the expected threshold for theexistence of a multi-path. As shown in the example of FIG. 3, twosignificant peaks are detected, L1 above the level L and L2 just belowthe level L. Data of both of these peaks L1 and L2 are stored in thestore P₁ as candidate multi-path positions.

As the correlation process continues, the correlators C₀, C₁, C₂, C₃ . .. C_(N) next make multiple correlation measurements over a secondintegration period T_(P2) in relation to the CPICH channel of a secondcell CELL 2 to determine corresponding candidate multi-path positionswhich are stored in the store P2. The same correlation process then runsfor each of successive integration periods T_(P3), T_(P4) . . . T_(PS)in relation to the CPICH channel of 3rd, 4th . . . 5th cells CELL 3,CELL 4 . . . CELL S to determine corresponding candidate multi-pathpositions and to store these in respective stores P₃, P₄ . . . P_(S).

At this point, the multi-path detector has analysed the candidatemulti-paths in S cells over a time period ST_(P), assuming that theintegration periods T_(P1), T_(P2), T_(P3), T_(P4) . . . T_(PS) are allequal to T_(P). It will be appreciated that T_(P)=T_(INT)+NΔ, whereT_(INT) is the period over which each correlator operates to generate acorrelation measurement. Therefore, the period T_(INT), delay Δ andnumber of correlators N are all selected in relation to the number ofcells S to ensure that the multi-paths are determined for the requirednumber of cells S in a predetermined time ST_(P).

Having measured candidate multi-paths for each of the cells S in a firstmeasurement cycle T1, as shown in FIG. 4, the correlators C₀, C₁, C₂, C₃. . . C_(N) then repeat the whole process in a second measurement cycleT2 to determine a second set of candidate multi-path positions for eachcell which are also stored in the respective stores P₁ to P_(S) holdingthe first candidate multi-path positions. The first and second set ofcandidate multi-path positions stored in each store P₁ to P_(S) arecompared with one another by the processor A to produce a modified setof candidate multi-path positions, some candidate positions beingconfirmed and others being rejected. This process of modification ofcandidate multi-path positions to produce a modified set of multi-pathpositions with a higher probability of correctness, continues insuccessive measurement cycles. In this way, any lower level ofprobability accepted for determining individual candidate multi-pathpositions, perhaps because of a shorter than optimum period T_(INT), iscompensated for by subsequent repeat measurements.

1-5. (canceled)
 6. A method of multi-path searching in a cellularnetwork in which a multi-path searcher is provided which carries out acorrelation process over a predetermined integration period in relationto a first pilot signal received in connection with a first cell of thecellular network to generate first multiple correlation measurementscorresponding to a plurality of time delays in the signal, analysingsaid first multiple correlation measurements in relation to saidplurality of time delays to identify multi-path positions in the signal,characterised in that information of said multi-path positions over afirst predetermined integration period is stored as a first set ofcandidate multi-path positions and compared with a second set ofcandidate multi-path positions derived from second multiple correlationmeasurements corresponding to said plurality of time delays made by themulti-path searcher over a second predetermined integration period inrelation to the first pilot signal, so as to confirm or reject candidatemulti-path positions and define modified candidate multi-path positions.7. A method as claimed in claim 6 in which the multi-path searchercarries out a correlation process in each of successive integrationperiods over a first cycle of integration periods and identifies a firstset of candidate multi-path positions during each integration period inrelation to the pilot signal of a respective different cell, and inwhich the multi-path searcher repeats said correlation process in eachof successive integration periods over a second cycle of integrationperiods to identify a second set of candidate multi-path positionsduring each in relation to the pilot signal of the same set of cells asduring the first cycle of integration periods, corresponding first andsecond sets of candidate multi-path positions relating to the same cellbeing compared to determine modified candidate multi-path positions forthe training signal of each cell.
 8. A multi-path searcher formulti-path searching in a cellular network by carrying out a correlationprocess over a predetermined integration period in relation to a firstpilot signal received in connection with a first cell of the cellularnetwork to identify the position of multi-paths in the signal,characterised in that storage is provided to store information of saidmulti-path positions over a first predetermined integration period as afirst set of candidate multi-path positions and to store a second set ofcandidate multi-path positions derived from multiple correlationmeasurements made subsequently by the multi-path searcher over a secondpredetermined integration period in relation to the first pilot signal,and a comparator provided to compare said first and second candidatesets of multi-path positions so as to confirm or reject candidatemulti-path positions and define modified candidate multi-path positions.9. A mobile handset incorporating a multi-path searcher as claimed inclaim
 8. 10. A computer program adapted to carry out the multi-pathsearching method of claim 6 when installed in a mobile handset.